Semiconductor light-emitting device

ABSTRACT

An insulation film  150  made of SiO 2  is formed on a p-type layer  106,  and a multiple thick film positive electrode  120,  which is a metal film formed through metal deposition, is formed on the insulation film  150  and on the p-type layer  106  at the central portion of which has a window and is exposed. The insulation film  150  has a thickness of one fourth multiple of emission wavelength. Thickness of the insulation film  150  is generally determined by multiplying one fourth of intramedium emission wavelength by an odd number. By interference effect, directivity of radiated light along the optical axis direction can be improved.

TECHNICAL FIELD

The present invention relates to a light-emitting semiconductor devicewith improved directivity. For example, a light-emitting semiconductordevice of the present invention can be used as an optical transmitter ina communication system using a plastic optical fiber (POF).

BACKGROUND ART

Basically, a light-emitting diode of a surface emitting type hasdouble-hetero structure which comprises, for example, a clad layer, anemission layer, and a clad layer deposited in sequence on a substrate.Especially, a flip-chip type light-emitting device comprises a metalelectrode on the uppermost surface, and electric current is supplied tothe emission layer uniformly from the electrode.

DISCLOSURE OF THE INVENTION

A conventional light-emitting diode of a surface emission type, however,has an undesirable directivity of optical axis direction, and a problempersists in that it is difficult to employ the conventionallight-emitting diode to optical communications using optical fibers.

The present invention has been accomplished in order to overcome theaforementioned drawbacks. Thus, an object of the present invention is toimprove directivity of a light-emitting diode of a surface emittingtype. By improving directivity, a light-emitting diode of a surfaceemitting type can be applied to optical communications.

In order to overcome the above-described drawbacks, the followings maybe useful. That is, a first aspect of the present invention provides alight-emitting semiconductor device which emits lights from a substrateside, comprising: an emission layer; a metal film which reflects lights;and a transparent insulation film which is sandwiched between anon-emission part of the emission layer and the metal film, wherein themetal film supplies electric current only to the emission part of theemission layer, and an interval of the emission layer and the metal filmis determined by multiplying by an odd number one fourth of anintramedium emission wavelength in the interval.

Because the interval between the non-emission part of the emission layerand the reflective metal film is arranged to be an odd number multipleof one fourth of an intramedium emission wavelength in the interval,each phase of emitted lights heading toward the metal film becomesidentical owing to multiple reflection between the metal film and theemission layer. As a result, lights converged on the direction verticalto the emission plane are emitted, which improves directivity of thedevice.

A second aspect of the present invention is a light-emittingsemiconductor device according to the first aspect, wherein theinsulation film comprises at least one selected from an oxygen compoundand a nitrogen compound.

A desirable insulation film can be obtained by using at least one of atransparent oxygen compound and nitrogen compound.

A third aspect of the present invention is a light-emittingsemiconductor device according to the second aspect, wherein theinsulation film comprises at least one selected from SiO₂ and TiO₂ andthe nitride compound comprises at least one selected from AlN and SiN.

A desirable insulation film can be obtained by using at least one of atransparent oxygen compound and nitrogen compound of the third aspect.

A fourth aspect of the present invention is a light-emittingsemiconductor device according to any one of the first to third aspects,wherein the light-emitting semiconductor device is a light-emittingdevice of Group III nitride compound semiconductor.

By forming the light-emitting semiconductor device using Group IIInitride compound semiconductor, lights having wider range of emissionwavelength, or green, blue, and ultraviolet lights, can be obtained.

A fifth aspect of the present invention is a light-emittingsemiconductor device according to any one of the first to fourthaspects, wherein the light-emitting semiconductor device is connected toa plastic fiber.

By using the light having high directivity which is obtained in thepresent invention incident on a plastic fiber, short-distance and simplecommunication system can be provided.

Although semiconductor materials of the above-described light-emittingsemiconductor device are not limited, a light-emitting semiconductordevice made of a Group III nitride compound semiconductor may compriselayers each of which is made of at least a group III nitride compoundsemiconductor, or a binary, ternary, or quaternary semiconductor havingarbitrary compound crystal proportions and represented byAl_(x)Ga_(y)In_((1−X−y))N(0≦x≦1; 0≦y≦1; 0≦x+y≦1). In the presentspecification, the “group III nitride compound semiconductor” alsoencompasses semiconductors in which the Group III elements are partiallysubstituted by boron (B), thallium (Tl), etc. or in which nitrogen (N)atoms are partially substituted by phosphorus (P), arsenic (As),antimony (Sb), bismuth (Bi), etc.

When the group III nitride group compound semiconductor layer has n-typeconductivity, Si, Ge, Se, Te, and C may be doped as n-type impurities.And as p-type impurities, Zn, Mg, Be, Ca, Sr, and Ba can be used.

A substrate, on which semiconductor layers are deposited, can be made ofsapphire, silicon (Si), silicon carbide (SiC), spinel (MgAl₂O₄), ZnO,MgO, other inorganic crystal substrate, gallium nitride (GaN) and othergroup III nitride group compound semiconductor.

In order to form the semiconductor layers through crystal growth,molecular beam epitaxy (MBE), a metal organic vapor phase deposition(MOCVD), Halide vapor phase epitaxy (Halide VPE), and liquid phaseepitaxy are useful.

In order to improve reflectivity of lights, the reflective metal filmcan comprise at least one selected from Al, In, Cu, Ag, Pt, Ir, Pd, Rh,W, Mo, Ti, and Ni and an alloy including at least one or more metals ofthese metals.

Through employment of the aforementioned aspects of the presentinvention, the aforementioned drawbacks can be overcome effectively andrationally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a light-emitting semiconductor device 100according to a first embodiment of the present invention.

FIG. 2 is an explanation view showing directivity characteristics of thelight-emitting semiconductor device according to the first embodiment ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will next be described withreference to the drawings. Characteristic features of the presentinvention have been described above, and the present invention is notlimited to the below-described specific embodiments.

FIRST EMBODIMENT

FIG. 1 is a sectional view of a flip-chip type semiconductor device 100of the present embodiment. The semiconductor device 100 is an example ofa light-emitting diode using a group III nitride compound semiconductor.About 20 nm in thickness of buffer layer 102 made of nitride aluminum(AlN) and about 4.0 μm in thickness of high carrier concentration n⁺layer 103 made of silicon (Si) doped GaN are formed on a sapphiresubstrate 101 in sequence. An emission layer 104 which has multiplequantum-well (MQW) structure comprising GaN and Ga_(0.8)In_(0.2)N isformed on the layer 103. About 60 nm in thickness of p-type layer 105made of magnesium (Mg) doped Al_(0.15)Ga_(0.85)N is formed on theemission layer 104. And about 15 nm in thickness of p-type layer 106made of magnesium (Mg) doped GaN is formed on the p-type layer 105.

On the p-type layer 106, a SiO₂ insulation film 150 is formed. On theinsulation film 150 and a portion of the p-type layer 106 which isexposed at a window formed almost at the center of the insulation film150, a multiple thick film positive electrode 120 is formed as a metalfilm by metal deposition. Thickness of the insulation film 150 isarranged to be an odd number multiple of one fourth of an intramediumemission wavelength in the insulation film 150. For example, when theemission wavelength is 430 nm, thickness of the insulation film 150 isarranged to be 107.5 nm. Accordingly thickness of the insulation film150 may be designed in relation to the emission wavelength. Generally,it is designed to be an odd number multiple of one fourth of theintramedium emission wavelength in the insulation film 150.Interferential action at this time improves the directivity of theemitted light in the optical axis direction. On the n⁺ layer 103, anegative electrode 140 is formed. The multiple thick film positiveelectrode 120 which functions as a reflective metal film comprises threelayers: about 0.3 μm in thickness of first metal layer 111 which is madeof at least one from rhodium (Rh) and platinum (Pt) and contacts to thep-type layer 106; about 1.2 μm in thickness of second metal layer 112which is made of gold (Au) and is formed on the upper portion of thefirst metal layer 111; and about 30 Å in thickness of third metal layer113 which is made of titanium (Ti) and is formed on the upper portion ofthe second metal layer. On the contrary, the negative electrode 140comprises two layers, about 175 Å in thickness of vanadium (V) layer 141and about 1.8 μm in thickness of aluminum (Al) layer 142, which aredeposited on the exposed portion of the n⁺ layer 103 of high carrierconcentration in sequence.

A SiO₂ protection film 130 is formed between the multiple thick filmpositive electrode 120 and the negative electrode 140 formed asexplained above. The protection film 130 covers the n⁺ layer 103 whichis exposed to form the negative electrode 140, the sidewall of theemission layer 104, the sidewall of the p-type layer 105, the sidewalland a portion of the upper surface of the p-type layer 106, thesidewalls of the first metal layer 111 and the second metal layer 112,and a portion of the upper surface of the third metal layer 113. Thepart of the protection film 130 made of SiO₂ film which covers the thirdmetal layer 113 has a thickness of about 0.5 μm.

The window A of the insulation film 150 is formed according to thefollowing processes. After the insulation film 150 is uniformly formed,a mask is formed through deposition of photoresist layer, exposure andpatterning. Then the insulation film 150 is etched to expose a portionof the p-type layer 106.

The metal film 120 has a multiple layer structure. Alternatively, themetal film 120 may have a single layer structure. The metal layer alsofunctions as an electrode which provides electric current to theemission layer 104. Alternatively, an electrode which provides electriccurrent to the emission layer 104 may be formed separate from the metalfilm which functions as a reflection layer. Further the materials arearbitrary, so long as metal materials having high reflectivity areselected to form the metal film according to emission wavelength.

The multiple thick film positive electrode 120 is formed on the windowA. Because the multiple thick film positive electrode 120 contacts tothe p-type layer 106, electric current flows to the downward only in thewindow A region. As a result, in the emission layer 104, the region inwhich the electric current flows becomes an emission region 108 and theother area of the emission layer 104 is a non-emission region 109. Andthe insulation film 150 exists only on the upper portion of thenon-emission region 109.

According to that structure, some light emitted from the emission region108 is outputted from the sapphire substrate 101 and some light whichforwards to the multiple thick film positive electrode (metal film) 120side is reflected by the metal layer. On the contrary, light radiatedfrom the emission region 108 in an oblique direction directs to themultiple thick film positive electrode (metal film) 120 and reflected bythe metal film 120. Further, light radiated from the emission region 108in an oblique direction is further reflected at an interface of eachlayer and an interface between the sapphire substrate and the outerspace of it, directs to the multiple thick film positive electrode(metal film) 120, and is reflected by the metal film 120. As a result,because thickness of the insulation film 150 is arranged to be an oddnumber multiple of one fourth of the intramedium wavelength, directivityof the optical axis direction is improved by interference of lights asshown in FIG. 2. Accordingly, by installing a fiber cable along theoptical axis direction, the device can be applied to opticalcommunications. Especially, the light-emitting semiconductor device ofthe present invention is useful for a comparatively shorter range ofoptical communications using plastic optical fiber (POF).

MODIFIED EXAMPLE

The present invention is not restricted to the above embodiments, andmany variations are possible. For example, in the first embodiment, aGaN-type semiconductor layer is used as a Group-III nitride compoundsemiconductor element, but a layer comprising Ga_(x)In_(1−x)N (such asGa_(0.92)In_(0.08)N) and the like or a ternary or quaternary compound ofthe elements Al, Ga, In, and N having a desired mixed crystal ratio maybe used. More specifically, a ternary (GaInN, AlInN, AlGaN) orquaternary (AlGaInN) Group-III nitride compound semiconductor expressedby the general formula Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1)can be used. A portion of the N in these compounds can be replaced by aGroup-V element such as P or As.

For example, when layers of a Group-III nitride compound semiconductorare formed on a sapphire substrate, in order to obtain a product ofimproved crystallinity, it is preferable to form a buffer layer so as tocorrect lattice misfit with the sapphire substrate. It is alsopreferable to provide a buffer layer when using a different type ofsubstrate. As a buffer layer, a Group-III nitride compound semiconductorwhich is formed at a low temperature such as Al_(x)Ga_(y)In_(1−x−y)N(0≦x≦1, 0≦y≦1, 0≦x+y≦1) and more preferably Al_(x)Ga_(1−x)N (0≦x≦1) isused. There may be a single such buffer layer, or multiple layers havingdifferent compositions may be used. A method of forming the buffer layermay be one which forms the buffer layer at a low temperature of 380–420°C., or the buffer layer may be formed by MOCVD at a temperature in therange of 1000–1180° C. In addition, high purity metallic aluminum andnitrogen gas can be used as source materials, and a buffer layercomprising AlN can be formed by reactive sputtering using a DC magnetronsputtering apparatus. In the same manner, a buffer layer expressed bythe general formula Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1, 0≦x+y≦1, withthe composition ratio being arbitrary) can be formed. Furthermore, it ispossible to use the vapor deposition method, the ion plating method, thelaser abrasion method, or the ECR method. Formation of the buffer layerby physical vapor deposition is preferably carried out at a temperaturein the range of 200–600° C. More preferably it is carried out at atemperature in the range of 300–600° C. and still more preferably in therange of 350–450° C. When a physical vapor deposition method such asthese sputtering methods is used, the thickness of the buffer layer ispreferably in the range of 100–3000 Angstrom. More preferably it is inthe range of 100–400 Angstrom, and most preferably it is in the range of100–300 Angstrom. Multiple layers can be formed by a method in which alayer comprising Al_(x)Ga_(1−x)N (0≦x≦1), for example, and a GaN layerare alternatingly formed, a method in which layers having the samecomposition are alternatingly formed with a forming temperature of atmost 600° C. and at least 1000° C., and the like. These methods can ofcourse be combined with each other, and the plurality of layers may beformed by laminating three or more Group-III nitride compoundsemiconductors of the formula Al_(x)Ga_(y)In_(1−x−y)N (0≦x≦1, 0≦y≦1,0≦x+y≦1). In general, a buffer layer is non-crystalline, and anintermediate layer is a monocrystal. A buffer layer and an intermediatelayer may be formed in a single cycle, or they may be formed inplurality of cycles, and cycles may be repeated any number of times. Themore repetitions the better is the crystallinity.

A high temperature growth buffer layer may be formed on a lowtemperature growth buffer layer, and the Group-III nitride semiconductorwhich is the main body may be formed on the high temperature growthbuffer layer.

In the buffer layer and the upper layers formed of a Group-III nitridecompound semiconductor, a portion of the composition of the Group-IIIelements can be replaced by boron (B) or thallium (Tl), and a portion ofthe nitrogen (N) can be replaced by phosphorus (P), arsenic (As),antimony (Sb), or bismuth (Bi). It is also possible to perform dopingwith these elements to an extent that the elements do not appear in thechemical compositional formula. For example, to the Group-III nitridecompound semiconductor Al_(x)Ga_(1−x)N (0≦x≦1), which does not includeindium (In) or arsenic (As) in its chemical formula, by performingdoping with indium (In) which has a larger atomic radius than aluminum(Al) or gallium (Ga), or with arsenic (As) which has a larger atomicradius than nitrogen (N), expansion strains of crystals due to nitrogenatoms coming out can be compensated by compressive strains, andcrystallinity can be improved. In this case, acceptor impurities easilyenter into the position of the Group-III element, and thus a p-typecrystal is obtained as grown.

When the buffer layer and the Group-III nitride compound semiconductorlayer are base layers formed by at least two cycles, each Group-IIInitride compound semiconductor layer can be doped with an element havinga larger atomic radius than the primary constituent element. Whenforming a light emitting element, it is generally preferable to use abinary or ternary Group-III nitride compound semiconductor.

When forming an n-type Group-III nitride compound semiconductor layer, aGroup-IV element or a Group-VI element such as Si, Ge, Se, Te, or C canbe added as an n-type impurity. In addition, a Group-II element such asZn, Mg, Be, Ca, Sr, or Ba or a Group-IV element can be added as a p-typeimpurity. These can be doped in a plurality of layers, or an n-typeimpurity and a p-type impurity can be doped in the same layer. AMg-doped GaN-type semiconductor implanted with Be can be changed to ahole density of 5.5×10¹⁶ to 8.1×10¹⁹/cm³ by annealing at 1100° C. for 60seconds. The activation energy of Mg is decreased to 170 mV byimplanting with Be. This is thought to be because Be breaks the bondsbetween Mg and hydrogen and then bonds with hydrogen. Accordingly, inorder to obtain a p-type layer, Be is preferably implanted in additionto acceptor impurities such as Mg.

The dislocations in the Group-III nitride compound semiconductor layercan be decreased by lateral epitaxial growth. At this time, although amask can be used, it is also possible to use a method which does notemploy a mask and in which trenches and posts are formed, and then alateral growth layer is formed over the trench. A method using trenchesand posts can form a spot-shaped or stripe-shaped trench and post on asubstrate and then form a gallium nitride type compound semiconductorover the trenches, and lateral growth can be carried out over thetrenches. It is also possible for there to be a gap present between thelateral growth layer and at least one of a layer beneath it and thesubstrate. When a gap is present, the introduction of strains due tostress is prevented, so crystallinity can be further improved.Conditions for lateral growth include a method in which the temperatureis elevated, a method in which the supply of gas of a Group-III elementis increased, and a method involving addition of Mg.

The p-type layers 106 to which the multiple thick film positiveelectrode 120 is joined preferably employs InGaN because doing soprovides a high hole density. An even higher hole density can beobtained by adding Be and Mg to the p-type layers 106. Mg is preferredas an acceptor impurity. For example, the composition is preferablyIn_(0.14)Ga_(0.86)N. It is possible to use a super lattice in the p-typelayers 106. For example, in order to increase the hole density of thelayer forming the multiple thick film positive electrode 120 and obtaingood ohmic properties, it is possible to employ a super latticecomprising p-type AlGaN/p-type GaN.

When successively forming layers of a Group-III nitride compoundsemiconductor on a substrate, as the substrate, it is possible to use aninorganic crystal substrate such as sapphire, silicon (Si), siliconcarbide (SiC), spinel (MgAl₂O₄), ZnO, MgO, or the like, a Group III-Vcompound semiconductor such as gallium phosphide or gallium arsenide, ora Group-III nitride compound semiconductor such as gallium nitride(GaN). Organic metal vapor phase growth (MOCVD or MOVPE) is preferred asa method of forming a Group-III nitride compound semiconductor layer,but molecular beam vapor phase growth (MBE), halide vapor phase growth(halide VPE), liquid phase growth (LPE), and the like may be used, anddifferent layers may be formed by different growth methods.

In the embodiment, a sapphire substrate 101 is used as a substrate, butthe above-described materials may be used as a substrate. For example,silicon carbide (SiC) maybe used as a substrate. In the embodiment, ap-n junction GaN group light-emitting device is explained.Alternatively, AlGaAs group, GaAlInP group and other materials may beused to form the device only if it is a solid light-emitting devicecomprising emission region of layers.

While the present invention has been described with reference to theabove embodiments as the most practical and optimum ones, the presentinvention is not limited thereto, but may be modified as appropriatewithout departing from the spirit of the invention.

The entire disclosures and contents of a Japanese Patent Application No.2002-153043, from which the present invention claims conventionpriority, are incorporated herein by reference.

1. A light-emitting semiconductor device which emits lights from asubstrate side, comprising: an emission layer; a metal film whichreflects lights; and a transparent insulation film which is sandwichedbetween a non-emission part of said emission layer and said metal film,wherein said metal film supplies electric current only to said emissionpart of the emission layer, and an interval of said emission layer andthe metal film is determined by multiplying by an odd number one fourthof an intramedium emission wavelength in the interval.
 2. Alight-emitting semiconductor device according to claim 1, wherein saidinsulation film is made of an oxygen compound or a nitrogen compound. 3.A light-emitting semiconductor device according to claim 2, wherein saidinsulation film comprises at least one selected from SiO₂ and TiO₂ andsaid nitride compound comprises at least one selected from AlN and SiN.4. A light-emitting semiconductor device according to claim 1, whereinsaid light-emitting semiconductor device comprises a light-emittingdevice of Group III nitride compound semiconductor.
 5. A light-emittingsemiconductor device according to claim 1, wherein said light-emittingsemiconductor device is connected to a plastic fiber.
 6. Alight-emitting semiconductor device according to claim 2, wherein saidlight-emitting semiconductor device comprises a light-emitting device ofGroup III nitride compound semiconductor.
 7. A light-emittingsemiconductor device according to claim 3, wherein said light-emittingsemiconductor device comprises a light-emitting device of Group IIInitride compound semiconductor.
 8. A light-emitting semiconductor deviceaccording to claim 2, wherein said light-emitting semiconductor deviceis connected to a plastic fiber.
 9. A light-emitting semiconductordevice according to claim 3, wherein said light-emitting semiconductordevice is connected to a plastic fiber.
 10. A light-emittingsemiconductor device according to claim 4, wherein said light-emittingsemiconductor device is connected to a plastic fiber.